Field effect transistors (FETs) are three terminal devices that are commonly used in radio frequency (RF) and microwave electronic circuitry to amplify signals. The three terminals of a FET include a gate, a source and a drain terminal. The gate is commonly used as the input of the FET and the drain is commonly used as the output of the FET. A voltage applied to the gate of the FET creates an electric field that controls the resistance between the FET's drain and source terminals. For a constant supply voltage the resulting resistance between the source and drain determines an electrical current. A small voltage applied to the gate of the FET is capable of causing a large change in output voltage by controlling the current flowing from drain to source. This is the basic operation of amplification.
During operation, biasing a FET is performed by applying a selected voltage to the gate terminal. The bias voltage defines a resistance between the source and drain that results in a DC or quiescent current that flows between the drain and the source. A time varying voltage signal applied to the gate causes current between the drain and source current to vary about its quiescent value.
In practice, the DC gate voltage required to achieve a desired quiescent drain current varies from FET to FET due to fabrication tolerances. As a result of these variations that occur during fabrication, each FET is tested to identify the required bias voltage needed to achieve a desired or target quiescent current for that particular FET. Once the required gate bias voltage is identified, additional circuitry may be implemented as a gate ladder to produce a predetermined voltage. For example, a voltage of negative 2.5 volts (−2.5V) may be applied to the gate terminal of the FET to set the desired FET's quiescent current.
Radar applications utilize FETs to amplify signals. Radar applications require that FETs be pulsed between an “ON” state, where current is allowed to flow between the drain and the source, and an “OFF” state, where no appreciable current flows between the drain and source terminals. Conventionally, FETs formed from semiconductors such as gallium arsenide (GaAs) are pulsed by placing a switch between the drain and the power supply and operating the switch to control the flow of drain-source current in a process called drain pulsing. The switches required for drain pulsing must be capable of switching on and off large drain currents with low losses. Accordingly, these switches are typically large, expensive, slow and add complexity, thereby making drain pulsing less than optimal for high-frequency applications like radar.
Gate pulsing refers to the pulsing operation of a FET by varying the bias voltage supplied to the gate terminal to control the drain to source current. Gate pulsing is not suitable for GaAs and similar semiconductor devices because of their low gate to drain breakdown voltages. Newer semiconductor technologies, such as gallium nitride (GaN) and silicon carbide (SiC) have higher gate to drain breakdown voltages and therefore, may be used for gate pulsing. However, this typically requires an additional power source for providing a pulsing bias voltage to the gate terminal, thereby adding complexity and cost.
Alternative solutions for pulsing FETs which address the above challenges are desired.